The present invention is related to a method for producing micromachined devices for use in Microelectromechanical Systems (MEMS). The present invention is also related to the devices obtained by said method and the use of said devices.
In Microelectromechanical Systems (MEMS), also known as Microsystems or Micro Machined Systems, devices are often used the production of which is based on the method technology developed in semiconductor processing, combined with specific MEMS technology. In MEMS technology, structures such as beams or cavities, can be added to micro-electronic circuitry. Compared to standard semiconductor devices, the mechanical properties of these micromachined devices are subject to very demanding criteria, in terms of breaking behaviour, weight, vibration resistance, etc., which impose higher standards on the processing of these devices.
To produce certain types of micromachined devices, the method of bulk micromachining is used, wherein wells are etched in a semiconductor wafer, leaving membranes, openings or beams. This device created include, for example, pressure sensors, accelerometers, inclinometers, or optical devices.
In the case of bulk micromachined accelerometers, for example, openings are etched in the semiconductor wafer. Often, these openings are large and narrow, resembling elongated cracks in the wafer.
In general, it is true that cavities or openings are formed in micromachined devices, which are large and deep in comparison to openings normally defined in semiconductor processing, such as contact or via holes, which have a diameter typically less than 5 xcexcm and a depth less than 2 xcexcm. Such large openings or cavities are responsible for a weakening of the wafer and increase the chance of the wafer breaking under the influence of stresses induced, for example, during processing.
It is known in the state of the art to exploit certain qualities related to the crystal structure of a semiconductor, such as in a Si wafer. For example, the etching of openings parallel to the  less than 100 greater than  direction is beneficial in the case of anistropic wet etching steps. This is described in documents such in EP-A-0658927 and U.S. Pat. No. 4,969,359, for example. However, narrow openings parallel to the  less than 100 greater than  direction tend to weaken the wafer considerably, and increase the danger of unintended breaking by cleavage during processing.
In document EP-A-0562880, a semiconductor infra-red emitting device or LED on a substrate is described, the substrate being slanted with respect to the LED stack. The slanted orientation of the devices on the substrate is meant to obtain cleavage of the substrate in a predefined way. This document is not related to avoiding cleavage and thus mechanical failure, but to obtaining this cleavage. Neither is this document related to the production of MEMS, comprising openings or cavities. It is merely a way of dividing a wafer into separate devices by way of cleavage. The same is true of the document DE-A-3435138, which describes a way of obtaining single chips by cleavage, with a reduced risk of splitting a wafer along a second direction perpendicular to the intended direction.
U.S. Pat. No. 4,278,987 describes a semiconductor device prepared according to a method wherein a dent is etched and subsequently filled, said dent having a polygonal shape having main sides parallel to a direction along  less than 100 greater than  or inclined within 25xc2x0 with respect to  less than 100 greater than . The purpose for this slanted position is not for mechanical strength purposes, but for epitaxial growth and planarization purposes. Devices according to this document are not MEMS.
Document JP-A-03219618 is related to cleaving wafers for SEM inspection. Crystal structure orientation is used to obtain cleavage through a device, not to prevent it.
The present invention aims to provide a method for producing micromachined devices which have a higher resistance to crack propagation during and after processing.
A further aim of the present invention is to provide micromachined devices having improved resistance to crack propagation.
The present invention is related to a method for producing micromachined devices for use in Microelectromechanical Systems (MEMS), comprising the steps of:
providing a crystalline wafer,
processing from said wafer at least one micromachined device comprising at least one elongated opening and/or cavity, having a longitudinal axis so that said longitudinal axis is at an angle to a direction which lies along the intersection of the front plane of the wafer and a cleavage plane, said cleavage plane being defined as a plane along which cleavage of the wafer is most likely to occur.
The present invention is related in particular to a method, wherein said wafer has the shape of a circular disc, with at least one part cut off along a chord of said circular disc, the longest of said chords being called xe2x80x98the flatxe2x80x99 of said wafer.
According to a first preferred embodiment of the present invention, said flat is not parallel to said intersection.
According to a second preferred embodiment of the present invention, said flat is parallel to said intersection.
In a preferred embodiment of the present invention, said wafer is a silicon wafer, whose front and back surfaces are oriented along a plane of the {100} family and wherein said cleavage plane is a plane belonging to the {111} or the {110} family. In these latter cases, said angle between said longitudinal axis of said opening and/or cavity and said direction is less than 45xc2x0.
In the embodiment wherein said flat is oriented along said intersection, the method according to the invention comprises the steps of:
subjecting said wafer to a photolithography step, whereby a pattern is printed through a mask onto said wafer,
etching said wafer,
wherein said photolithography step comprises the step of rotating said mask over an angle, with respect to said wafer, or wherein said pattern is positioned at an angle with respect to said mask, or wherein said photolithography step comprises the step of rotating said wafer over an angle with respect to said mask.
According to the embodiment wherein said flat is oriented along said intersection, said photolithography step may comprise a contact printing step or a proximity printing step.
In the embodiment wherein said flat is not oriented along said intersection, the method according to the invention comprises the steps of:
subjecting said wafer to a photolithography step, whereby a pattern is printed through a mask onto said wafer,
etching said wafer,
wherein said photolithography step may comprise a contact printing step, a proximity printing step or a number of projection printing steps.
The present invention is also related to a micromachined device for use in Microelectromechanical Systems, said device being produced according to the method of the invention.
The present invention is also related to the use of a micromachined device, said device being produced according to the method of the invention.